Questions about a readonly @property in ARC

Question

In my interface (.h) file, I have

@property(readonly) NSString *foo;

and in my implementation (.m) file, I have

@synthesize foo;

With ARC turned on, the compiler gives me this error: Automatic Reference Counting Issue: ARC forbids synthesizing a property of an Objective-C object with unspecified ownership or storage attribute.

The error goes away if I add a strong, weak, or copy to the property. Why is this? Why would there be any differences between these things for a read-only property, what are those differences, and why does the programmer have to worry about them? Why can’t the compiler intelligently deduce a default setting for a read-only property?

Another question while I’m at it: strong, weak, or copy are the only things that make sense in ARC, right? I shouldn’t be using retain and assign anymore, should I?

Answer 1

You've declared a @property that doesn't have a backing ivar. Thus, when the compiler sees @synthesize, it tries to synthesize a backing ivar for you. But you haven't specified what kind of ivar you want. Should it be __strong? __weak? __unsafe_unretained? Originally, the default storage attribute for properties was assign, which is the same as __unsafe_unretained. Under ARC, though, that's almost always the wrong choice. So rather than synthesizing an unsafe ivar, they require you to specify what kind of ivar you want.